Linear induction motors are well known in the art. In some applications the wound portion, or primary, of the linear induction motor is connected to a fixed voltage, fixed frequency power supply. Power is supplied to the motor so that the motor produces a propelling force, accelerates and reaches a steady state speed. The steady state speed is determined by motor characteristics, mechanical characteristics of the load and power supply parameters. Motors designed to operate in such applications are not designed for minimum weight, minimum size and minimum power consumption. Rather, the governing considerations are usually matching motor characteristics to load and minimizing cost. Control of motors used in such applications is usually limited to turning them on and off. An optimized control strategy would not normally be used in such an application.
However, in some environments employing linear induction motors, such as the mass transit vehicle environment, it is desirable to ensure that the linear induction motor the primary of which is carried by the vehicle, develops maximum thrust throughout the course of its operation. By operating the linear induction motor in this manner, it is possible to minimize the required size and weight of the primary while ensuring that the motor develops sufficient thrust for all possible load conditions.
Conventional controllers used to operate linear induction motors in this desired manner make use of memory look-up tables which store predetermined operating parameters for the linear induction motor. The stored operating parameters consist of various motor speeds and the corresponding supply voltage frequencies at which maximum thrust should be developed by the linear induction motor at those speeds. The frequency parameters stored in the memory look-up table are determined through experiment and are dependent upon many factors including the physical construction of both the linear induction motor primary and the linear induction motor secondary, the ambient temperature, the temperature of the linear induction motor primary, as well as the power supply voltage. The parameters are therefore determined with the motor operating under a set of nominal conditions.
During operation of the linear induction motor, the prior art controller consults the look-up table to find the power supply frequency which corresponds to the current operating speed of the linear induction motor. The controller then adjusts the power supply frequency to correspond with that found in the look-up table so that the thrust developed by the linear induction motor primary changes. Based on the speed and frequency pairs stored in the look-up table, the thrust developed by the linear induction motor, in view of the power supply frequency change, should be the maximum thrust point of the linear induction motor.
Although this prior art method of controlling a linear induction motor achieves some success, problems exist in that the thrust developed by the linear induction motor is influenced by a number of factors in addition to the motor operating speed. These additional factors may differ from the nominal conditions under which the motor was operated during formation of the look-up table. For example, the ambient temperature, the temperature of the linear induction motor primary or the resistance of the linear induction motor reaction rail may change causing a shift in the thrust versus frequency characteristic curve of the linear induction motor any given speed. This resultant shift in the characteristic curve causes a shift in the supply voltage frequency at which the linear induction motor develops maximum thrust.
If these changes from the nominal conditions occur and a shift in the thrust versus frequency characteristic curve results, the prior art controllers will select a power supply frequency in accordance with the values in the look-up table that results in the linear induction motor developing thrust having a lesser magnitude than its maximum thrust capability at that motor speed. Depending on the type and the magnitude of the changes in the motor operating parameters, the difference between the thrust developed by the linear induction motor and its maximum thrust capability may vary significantly. In view of this potential difference, linear induction motor primaries have typically been over-dimensioned to ensure that the linear induction motor develops sufficient thrust for all possible load requirements despite the changes in the operating parameters of the linear induction motor.
It is therefore an object of the present invention to obviate or mitigate the above-mentioned disadvantages by providing a novel controller for a linear induction motor.